This invention relates generally to heating apparatuses and more specifically to a multiple stage hot water supply heater.
It is known to provide commercial, gaseous and liquid fossil fuel heaters which use multiple burners in a combustion chamber for heating water. The heated water typically runs through tubes which are also disposed in the combustion chamber. The burners are of a premix air and gas variety, and one or more fans supply air to an air chamber in communication with orifices in the burners.
It is desirable to vary or reduce the firing or burning rate of the burners in order to match the load placed on the appliance. This can be done by varying the input to the burners, by turning off individual burners, or by a combination of modulation and discrete step firing rate reduction. The modulation approach varies the amount of fuel and/or air supplied to the burners rather than turning them only on or off. However, modulation usually requires expensive controls and monitoring equipment to insure safe and efficient operation. Furthermore, it would be very expensive to build an appliance with many individually controlled zones due to the complexity of controls necessary to properly balance the unit for the variable input construction.
The approach of turning off individual burners within one controlled zone upsets the balance but is otherwise fairly economical. The combustion chamber pressure imbalance can force the products of combustion to migrate from the firing burners toward the zones or areas of the non-firing burners and can recirculate back into the path of the fired burners. Thereafter, the products of incomplete combustion are allowed to escape from the unit, past the non-firing burners, which is unacceptable from an environmental emissions standpoint. In other words, the pressure zone above the non-firing burners (i.e., a flame and fuel are not present) is lower than that above the firing burners when one set of burners is not firing. This induces movement of unburned combustion products toward the lower pressure zone. Similarly, the pressure in the area below the non-firing burners can be lower than the areas where the burners are firing. This also induces movement of the uncombusted products from below the firing burners toward the lower pressure zone.